A poly(trimethylene terephthalate) (hereinafter abbreviated to PTT) is a polycondensation product of terephthalic acid and/or a lower alcohol ester of terephthalic acid with 1,3-propane diol (also termed trimethylene glycol, hereinafter abbreviated to PDO).
PTT fibers obtained by melt spinning a PTT have many excellent properties such as an astonishingly soft feeling, drapability, excellent stretchability, low temperature dye-affinity and weathering resistance. Conventional synthetic fibers such as conventional poly(ethylene terephthalate) (hereinafter abbreviated to PET) fibers and nylon 6 fibers have no such excellent properties.
A process comprising once pelletizing a prepolymer of a PTT obtained by melt polymerization, and heating the prepolymer in a pellet form without remelting the pellets to effect polymerization, namely, a production process of a PTT in which melt polymerization is combined with solid-state polymerization, has been known as a production process of a PTT.
Elementary reactions forming the polycondensation reactions of a PTT mainly include the following two types of reactions: the forward reaction of a chain growth reaction (the following formula (a1)) in which PDO is removed from two terminal hydroxyl groups; the backward reaction including a reaction (the following formula (a2), namely the reverse reaction of the formula (a1)) in which an ester portion is decomposed with undischarged PDO and a thermal decomposition reaction (the following formula (b)).

A PTT is more likely to suffer a thermal decomposition reaction than a PET and a poly(butylene terephthalate) (hereinafter abbreviated to PBT), each having a skeleton similar to that of a PTT. That is, kd in the above formula (b) is large. As a result, it is difficult to adequately increase the molecular weight of a PTT by melt polymerization alone. A process wherein solid-state polymerization (in which polymerization is carried out at temperature as low as up to the melting point) is carried out in combination is usually employed after melt polymerization.
However, various problems caused by the properties of the polymer arise in the production of a PTT.
A first problem is that a PTT tends to be thermally decomposed in a melt polymerization step. Because kd is large in the above formula (b), lowering of the molecular weight of a PTT is likely to take place at high temperature. Furthermore, carboxyl groups and allyl groups produced by thermal decomposition accelerate further thermal decomposition, which causes lowering of the whiteness and oxidation resistance stability of the polymer.
Inhibiting the thermal decomposition as much as possible therefore becomes an essential requirement for obtaining a PTT of high quality in the melt polymerization step of a PTT. However, known technologies are still insufficient to satisfy the requirement. Such thermal decomposition does not matter substantially in the production of a PET and a PBT. For a PET or a PBT, the thermal decomposition rate constant corresponding to kd is far smaller in comparison with a PTT, and the thermal decomposition hardly takes place. As a result, polymers of high quality can be produced by melt polymerization alone, and polymers each having a sufficiently high molecular weight can be obtained without a combination with solid-state polymerization. Accordingly, the first problem is one extremely specific to a PTT, and solution of the problem is difficult even when known information about a PET and a PBT is used.
A second problem is as follows: although solid-state polymerization inhibits the thermal decomposition and a PTT having a high molecular weight can be obtained, the polymerization rate sometimes becomes significantly low due to low polymerization temperature. It takes a significantly long time for known technologies to subject a PTT to solid-state polymerization, though the time differs depending on the molecular weight of a prepolymer and the molecular weight to be attained. A decrease in the productivity is therefore unavoidable. Moreover, although the polymerization temperature is low, thermal decomposition of a polymer is unavoidable to a certain degree when the solid-state polymerization is carried out at temperature close to 200° C. for a long period of time.
A third problem is as explained below. A PTT in a state of pellets tends to be cracked, and powdery material is likely to be produced when PTT pellets rub together during transportation, drying, solid-state polymerization, and the like of the PTT. Moreover, a cyclic oligomer formed during melt polymerization is contained in the PTT in a large amount. When PTT pellets and the powdery material are in a mixture, yarn breakage and fluff formation are likely to take place in the melt-molding step. Furthermore, because a cyclic oligomer is highly volatile, it is deposited around the spinning nozzle during the melt spinning step, and the deposits also cause yarn breakage and fluff formation.
The crack formation, generation of powdery material and deposition of a cyclic oligomer are problems specific to a PTT. A PET and a PBT each having a structure similar to a PTT substantially have no such problems.
When a PTT is produced by melt polymerization alone, the content of a cyclic oligomer in the PTT amounts to 1.6 to 3.5% by weight. In contrast to the PTT, the oligomer content in a PET is about 1% by weight. Moreover, the oligomer of a PET is mostly cyclic trimer. However, the cyclic oligomer of a PTT is mostly cyclic dimer, and as a result the cyclic oligomer has a small molecular weight. The cyclic oligomer is therefore greatly volatile and soluble in water. Accordingly, the problems of the cyclic oligomer in the production process of a PTT are far more serious in comparison with those of a PET.
Several production processes in which melt polymerization and solid-state polymerization are employed in combination have been known. However, no process that solves the above problems simultaneously has hitherto been known.
For example, Japanese Unexamined Patent Publication (Kokai) No. 8-311177 describes that when PTT pellets are subjected to solid-state polymerization at temperature close to 200° C. for a few hours in vacuum, the oligomer content amounts to 1% by weight or less. However, the above problems except for the problem of the oligomer are not recognized in the patent publication, and thus the patent publication does not describe methods of solving the problems.
In the specification of U.S. Pat. No. 2001/0056172 A1, a process for solid-state polymerizing PTT pellets in an amount from 10 to 15 mg is described. However, generation of powdery material and the problem of a cyclic oligomer are not recognized at all in the patent publication, and thus no suggestion related to methods for solving the problems is described therein.
Japanese Unexamined Patent Publication (Kokai) No. 2000-159875 discloses a process for obtaining a PTT of high grade by solid-state polymerizing a polymer obtained by melt polycondensing with a catalyst mixture of Ti and Mg in a specific state and having a low terminal vinyl group content, under reduced pressure or in an inert gas atmosphere. However, because Mg is used as a catalyst in the process, the PTT has a dull color tone, and the L* value is as low as about 60 to 70; the pellets have a poor color tone. Moreover, no suggestion about recognition of the above problems or a method for solving the problems is described therein.
The pamphlet of International Patent WO 97/23543 describes a process comprising dropping a molten PTT having a low polymerization degree on a hot plate without pelletizing, crystallizing the PTT at temperature from 60 to 190° C. to form a PTT in a solid state having an apparent fine crystal size of 18 nm or more, and then solid-state polymerizing the PTT. However, the TTT obtained by the process has a drastically uneven surface, and easily produces powdery material when rubbed together. The PTT is therefore estimated to have poor moldability. Moreover, the pamphlet includes no description about the color tone and improvement of the oxidation resistance stability, and neither describes nor suggests recognition of the problems about the moldability, crack formation and powdery material of PTT or solution methods of the problems.
The pamphlet of International Patent WO 98/23662 describes in Example 8 a process comprising pelletizing a PTT the terminals of which are sealed with a hindered phenol stabilizer, and solid-state polymerizing the pelletized PTT. The pamphlet of International Patent WO 99/11709 describes in Example 8 a process comprising pelletizing a PTT containing a phosphorus type stabilizer, and solid-state polymerizing the pelletized PTT. However, both pamphlets neither describe nor suggest recognition of the problems about the moldability, crack formation and powdery material of the PTT or solution of the problems.